Cellular Receptors

As discussed in greater detail in Chapter 3 the primary receptor for all HIVs and SIVs is CD4, which is present on the surface of CD4⁺ T lymphocytes, cells of the monocyte/macrophage lineage, and certain other cells (Dalgleish et al. 1984; Klatzmann et al. 1984). Infection of all these cell types in vitro can be blocked by some monoclonal antibodies to CD4 as well as a soluble version of CD4. Besides demonstrating the requirement of CD4 for infection, these observations suggested that it might be therapeutically useful to block the interaction of CD4 and gp120. However, such approaches have not proven to be clinically useful (for review, see Eiden and Lifson 1992; Chapter 12.

In addition to its role as the principal receptor for HIV, CD4 is crucial for the generation of immune responses because it is the natural ligand for major histocompatibility complex (MHC) class II molecules (Doyle and Strominger 1987; Gay et al. 1987; Chapter 12. When an antigen-presenting cell displays processed peptide antigens in association with the MHC class II molecules to the T-cell receptor complex on the CD4⁺ T lymphocyte, CD4 serves as an adhesion molecule that stabilizes the interaction. Signaling is achieved, at least in part, through protein kinase molecules bound to the cytoplasmic domain of CD4. As discussed below (see Immunopathogenic Mechanisms of HIV Infection), the binding of HIV gp120 to CD4 may be an important component of HIV-1 pathogenesis.

Although human CD4 is essential for HIV infection, it is not sufficient. Expression of human CD4 on rodent cells renders them capable of binding virus but still nonpermissive for fusion or infection (Maddon et al. 1986). In addition, although HIV can bind to CD4 on human brain and skin cells, fusion between the HIV envelope and the cell membrane fails to occur in these cells, indicating the absence of a cell surface component essential for fusion (Chesebro et al. 1990). These data, taken together with the resistance of certain CD4⁺ human cell lines to infection by HIV-1, suggested that secondary cell surface receptors were needed for viral entry.

The host-cell component, or coreceptor, sometimes referred to as the “fusion receptor,” was identified only recently. As described in more detail in Chapter 3 several receptors for chemokines—small proteins which serve as chemoattractants in inflammation—also act as coreceptors, permitting HIV infection of virtually any mammalian or avian cell that expresses human CD4 (Bates 1996; Moore et al. 1997). The two most important coreceptors are CXCR4 (also called fusin or LESTR) (Endres et al. 1996; Feng et al. 1996) and CCR5 (Akhatib et al. 1996; Choe et al. 1996; Deng et al. 1996; Doranz et al. 1996; Dragic et al. 1996). CXCR4 is the receptor for the chemokine SDF-1 (Bleul et al. 1996; Oberlin et al. 1996), whereas CCR5 serves as a receptor for the chemokines MIP-1 α and β as well as RANTES (Murphy 1996). As discussed below, not only do the coreceptors provide a crucial function for viral entry into cells, but they are also the principal determinants of tropism among CD4⁺ cells. The chemokines that bind to these receptors can block HIV-1 infection (Cocchi et al. 1995; Bleul et al. 1996; Oberlin et al. 1996), and a defect in the gene encoding CCR5 causes natural resistance to HIV infection (Dean et al. 1996; Liu et al. 1996; Paxton et al. 1996; Samson et al. 1996).

Several other cell surface molecules have been shown to function as receptors for HIV in vitro, although their relevance in vivo remains to be demonstrated. The Fc portion of immunoglobulins or complement receptors (FcR) can facilitate infection of cells of the monocyte/macrophage lineage by HIV-antibody complexes. The FcR can be expressed on the surface of human fibroblasts following infection with cytomegalovirus (CMV). This allows these cells to be infected by HIV immune complexes (McKeating et al. 1990). In addition, in the presence of complement, antibody-independent binding of opsonized HIV to complement receptors can result in cell infection (Boyer et al. 1991; Ebenbichler et al. 1991).

Infection of some CD4-negative cell types in vitro such as colonic epithelium can be mediated by galactosyl ceramide as indicated by inhibition by monoclonal antibodies and other studies (Bhat et al. 1991; Manca 1992; Yahi et al. 1992; Fantini et al. 1993). It should be pointed out that the level of HIV replication in these CD4^– cell lines is in general quite low compared to permissive CD4⁺ cells. Carbohydrate-mediated binding of the mannose residues of HIV gp120 to the endocytosis receptor on the surface of macrophages has been observed (Larkin et al. 1989). A membrane-associated mannose-binding lectin has also been implicated in CD4-independent HIV infection of human placental tissue (Curtis et al. 1992).

Host Range of Cellular Targets

All lentiviruses including HIV and SIV have a specific tropism for macrophages. Although immature cells of the monocyte/macrophage lineage can be infected by nonprimate lentiviruses, viral RNA expression is limited. As the cells mature and enter the peripheral blood as monocytes, viral gene expression increases. Once the monocytes differentiate into mature macrophages, infectious viral particles are produced (Gendelman et al. 1985). All lentiviruses can replicate in terminally differentiated, nondividing macrophages (Narayan et al. 1988). This requires the preintegration complex to enter the nucleus of the resting cell, and HIV, at least, appears to have special adaptations that allow the preintegration complex to transit the nuclear membrane (Chapter 5.

The receptor(s) that nonprimate lentiviruses use when they infect macrophages is generally not known, but the primary receptor is not CD4. HIV and SIV do use the CD4 molecule as the primary receptor for entry into the macrophages, since infection of these cells is blocked by certain anti-CD4 monoclonal antibodies. Thus, SIV and HIV use a receptor that is different from those of the other lentiviruses, yet they have retained a specific tropism for macrophages. The use of the CD4 molecule as the primary receptor allows HIV and SIV to have an expanded host-cell range that includes CD4⁺ T lymphocytes and cells of the monocyte/ macrophage lineage (Fauci 1988).

CD4⁺ T lymphocytes and macrophages differ in several important ways. HIV infection of CD4⁺ T cells in vitro leads to extensive cell death, whereas HIV infection of monocyte/macrophages produces a more limited cytopathicity in vitro. In CD4⁺ T cells, virions are released almost exclusively at the plasma membrane, whereas in monocyte/macrophages, HIV particles are often released within intracytoplasmic vacuoles (Orenstein et al. 1988; Gendelman et al. 1989). CD4⁺ T cells and monocyte/macrophages also differ in their ability to sustain replication of particular HIV isolates (see below, Viral Determinants of Cellular Tropism).

In contrast to the situation with macrophages, HIV and SIV require that CD4⁺ T lymphocytes be activated for optimal replication (McDougal et al. 1985; Zack et al. 1990, 1992). There is little or no replication of HIV and SIV in lymphocytes in resting peripheral blood mononuclear cell (PBMC) cultures. Since most lymphocytes in the blood are in a resting state, only a small fraction are suitable targets for infection. Cellular activation increases both the size of nucleotide pool needed for viral DNA synthesis and the level of transcription factors such as NF-κB that are important for the transcription of proviral DNA (Nabel and Baltimore 1987; Zack et al. 1990). Resting cells also express very low levels of CCR5 (Bates 1996), again making them unsuitable hosts at least for M-tropic HIV (for a discussion of M-tropic HIV, see next section).

Although relatively few HIV-infected monocytes are found in the peripheral blood, infected tissue-specific macrophages have been observed in a variety of organs including the brain and lung (for review, see Rosenberg and Fauci 1989a). SIV replication in macrophages of brain, lung, and other tissues is also associated with tissue-specific diseases in monkeys (Desrosiers et al. 1991). Similarly, the principal organs that are affected upon infection of tissue-specific macrophages by the nonprimate lentiviruses include the central nervous system, lungs, synovia, and the mammary gland (Narayan et al. 1988). Although infection of macrophages could be viewed as an evolutionary remnant that is relatively unimportant for the HIV/SIV life cycle, it is generally believed that infection of macrophages is important for the in vivo replication of primate lentiviruses and for their ability to cause disease. In particular, the first cells infected during mucosal transmission are likely to be of the macrophage lineage. The relative importance of macrophage infection in viral replication, persistence, and disease pathogenesis needs further clarification, particularly for the primate lentiviruses.

Viral Determinants of Cellular Tropism

The ability of HIV to infect different cell types varies from isolate to isolate. The first successful HIV isolations were performed in mitogen-stimulated PBMCs (Barré-Sinoussi et al. 1983; Popovic et al. 1984). Subsequent experiments showed that some HIV-1 isolates could be propagated in established T-cell lines. Later studies showed that certain HIV-1 isolates that did not grow in established cell lines could replicate in primary macrophage and T-cell cultures.

In general, two distinct types of HIV-1 have been identified based on the cells they replicate in in vitro. Viruses that replicate in T-cell lines, but not macrophages or monocytes, are referred to as T-tropic, whereas viruses with the complementary specificity are referred to as M-tropic. The tropism of the virus has been shown to be a function of the coreceptor used: M-tropic viruses can use only CCR5 for entry; T-tropic viruses use CXCR4 (Akhatib et al. 1996; Choe et al. 1996; Dragic et al. 1996; Feng et al. 1996). Tropism thus reflects the cell distribution of coreceptor expression. A few dual-tropic isolates capable of using both are also known (Doranz et al. 1996; Simmons et al. 1996). T-tropic viruses often cause infected cells to fuse with uninfected cells if the latter express both human CD4 and CXCR4; such viruses are referred to as “syncytium-inducing” (SI). It is important to keep in mind that all isolates can infect activated T cells freshly isolated from peripheral blood, which are present in PBMC cultures. Such cells express both CCR5 and CXCR4. Furthermore, cell tropisms are not fixed and can change when the virus is passaged in cell culture (for review, see Meltzer et al. 1990; Levy 1993).

Although infected lymphocytes and macrophages are both present within the blood of HIV-1-infected patients, infection of lymphocytes appears to predominate in this compartment (Psallidopoulos et al. 1989; Spear et al. 1990). However, both lymphocytes and macrophages are targets of HIV-1 in tissues. Certain tissue-specific disease manifestations of SIV and HIV infection such as encephalitis and interstitial pneumonia are associated with extensive infection of the macrophages (Kaaya et al. 1993; Simon et al. 1994). Given the substantial variation in the sequence of the viral genome that occurs in infection associated with SIV and HIV, it is not surprising that viruses with a spectrum of differing tropisms are often present within a single infected individual.

Recombinants between cloned T-tropic and M-tropic viruses have demonstrated that the env gene is the primary determinant of cell tropism for both HIV and SIV (for review, see Banapour et al. 1991; Mori et al. 1992, 1993; Sakai et al. 1992; Levy 1993). Variable region 3 (V3) of gp120 is a key component within env that determines cell tropism. In fact, a single point mutation within V3 can alter tropism (Takeuchi et al. 1991). The efficiency of replication and the ability to induce the syncytia formation are also affected by changes in the V3 loop (Cheng-Mayer et al. 1991; Hwang et al. 1991; de Jong et al. 1992; Fouchier et al. 1992). These properties of V3 are apparently exerted at the level of the coreceptor (Liu et al. 1990; O'Brien et al. 1990; Freed and Risser 1991; Choe et al. 1996; Cocchi et al. 1996). Changes in env outside of the V3 loop can influence the conformation of gp120, as well as the association of gp120 with gp41, which can affect host range (Stamatatos and Cheng-Mayer 1993). Residues outside of V3 can determine the ability of SIV to replicate efficiently in macrophages (Mori et al. 1993). Other elements, such as vpr, vpx, and the long terminal repeat (LTR), may also be determinants of the ability to replicate in macrophages (Banapour et al. 1991). It has been suggested, but not yet clearly demonstrated, that certain sequences of env may preferentially facilitate replication in macrophages of one tissue over another (Kodama et al. 1993).